Magnetic focussing device



March 4,1947. E, FRY 2,416,687

- MAGNETIC FOCUSSING DEVI-CE Filed um 30, 1944 4 Sheets-Sheet 1 INVENTOR E M FRY BY A TZ'ORNEV March 4,1947. I 2,416,687

MAGNETIC FOCUSS ING DEVICE Fi led March so, 1944 4 Sheets-Sheet 2 Y INVENTOR E M FRY March 4,1947. E, M. FRY 2,416,687.

MAGNETIC FOCUSSING DEVICE Filed March ,30, 1944 4 Sheets-Sheet 4 FIG. 8 4

IN VEN TOR E M FRY BY Wm flaz;

' ATTORNEY Patented M ar. 4,1947

UNITED STAT Es PATENT orm MAGNETIC FOCUSSING DEVICE I Eugene M. Fry, Matawan, N. J., assignortojBcll' Telephone Laboratories, Incorporated, New York ,.N..Y. a corporation of New York Application March 30, 1944, Serial No. 528.697

This invention relates to magnetic focuss'ing devices and more particularly to such devices employed in focussing the electronic beam in cathode ray oscillograph devices.

In the practical operation of cathode-ray oscillograph devices, a beam ,or electrons is projected from a source positioned at a considerable distance from a fluorescent screen on which the beam impinges in order thata wide sweep or' deflection control maybe obtained. In addition to 19 Claims. (Cl.'250161l the deflection control which may be magnetic or beam also decenters-the, spot, since the beam follows a spiral'path and a change in the focussing field alters the pitch of the spiral in which the electrons travel. This interaction causes numerous adjustments to be made during the warm- -up'D IfiOd of the cathode-ray tube and "under Varying temperature levels of operating conditions, such as at ground level and at high altitudes.

Moreover, a separate direct current source of sufi'icient wattage is required for these coils which is not always convenient on account of the increased weight of equipment, for example, in an airplane where space is limited. Many of these .difficulties are avoided in the focussing of the beam by the employment of a permanent magnet structure of small mass and concentrated magnetic flux pattern, such as an aluminum alloy magnet, although mounting and adjusting difii- .culties are still evident to adapt the permanent magnet to the cooperative functions with respect to the device to. influence constant definition and concentricit y. of the beam over a range of accelerating voltages of large magnitude.

The primary. objects of this invention are to increase the operating range and to improve the focussin'g characteristics of the magnet for use with cathode-ray oscillograph devices.

A specific object of the invention is tofacili;

tate the manipulation of the components of the magnet tosecure efllcient operation in the focussing and centering of the electron beam in cathode-ray devices.

Another object of the invention is to improve the flux field of the magnet whereby a large range of anode or accelerating voltagesfmayv be utilized to accomplish sharp definition of th beam on the screenoi the device. --A further object of the. invention. is'toe'fllciently control the centering of the beam over a wide radius of the screento overcome diverse alignment of the electron source with respect to the axis of the device and the center. of an indicator scale superimposed on the screen of the device. I v

A further object of the invention is to facilitate the-remote control of the flux field of the magnet .mounted around the device so that adjustment may be accomplished without disturbingthe device and the magnet structure usually enclosed in a protective casing. v

These objects and advantages are obtained in accordance with this invention by employing an alloy ring of high magnetic flux density as the main component of the focussing magnet structure and surrounding the ring with an adjustable shunt member which may be reciprocally movable with'respect to the pole-pieces of the ring to vary the strength of the'magnetic field up'to 200 gausses. An annular auxiliary plate on one end of the magnetic structureis adjustably movable in transverse directions to distort the field and influence the directional fiowof the electron V beam in the device whereby .contralization of the beam with respect to the axial line of the screen is produced in "a facile manner. This construction forms a unitary assembly which may be mounted on the neck of the cathode-ray tube and the controls extended by flexible cables to a front panel adjacent the viewing scale of the device.

The specific assembly of the unitarystructure involves a shunt ,member controlled reciprocally by a chaindrive coupled to a plurality ofscrew members extending through the shunt and located between. the shunt and magnet proper. Another form involves a. rotatable sleeve having arcuate slots therein with adjustable means in the slots to vary the relationship of the shunt with respect .to the magnet pole-pieces.

The specific construction of the centering component .of the structureinvolves movementof an annular plate on oneend of the magnet assembly by cam shafts operating'against tensioned springs disposed in opposite relation-to the cams to shift v the plate transversely with respect to the axis of the magnet structure. 7 ln a modified arrangement of this feature of the invention, the plate is controlled by radial shafts having extensions thereon located in transverse slots in the plate to shift the plate with respect to the axis of the magnet structure.

These and other features'of the invention are best understood by referring to the following I detailed description and the accompanying drawings.

Fig. '1 illustrates in a general side view the application of the magnet structure of this invention to the neck of a cathode-ray device having a fluorescent screen adjacent a view-ingplatetonsa panel and the controls mullntedpn the panelior adj usting the focussing antleentelziljgcqmpnnents of the magnet structure; v I

Fig. 2 is a perspective view of one embodiment of the invention utilizing the chain-driveifor. the

movement of the shuntand collateral shaftslfcr themovement of the centering plate member on the rear offlthe magnet with a portion of the shunt member "broken away to show the connection of one oithe travel :blockstto the-vertical turelshown in Fig, 5.;

Fig. .8 illustrates, ina perspective exploded view the ,components of the magnet assembly. of Figs.

.6 ,and-:'7;l.and

.Fli s. 9j and .a are diagrammaticviews of the .same .rnagnet assembly .With one of 'the ,two

extreme positions of wthe,cerlter.ingrring and shunt :Shqwndn dotted ,line,.r.espectively. 1

7 Referring toFi-gl ofllhe drawing, the magnetic iocussing and centering .device 1!! of this iinven- ,-tio.n"..is -.monnt.ed .on the .neck of a cathode-pray .tubeor .device ,Ll, having ,an electronglln 1.2 at

1 ,one ,end thereof disposedto theirear ofthemage netic focussmg device and adapted ;to,produce a narrow .loeam ofrelectrons which is prejected "to .a fluorescentscreen 13 on .the opposite .end of V the cathode-ray .device. ,The electron beam is magnetically focussed,towproducelsharp definition of the spot arriving at theiscreen and "the beam .is vradially shifted by the centerin plate to .correct external magnetic fields ,andmanufac- .ituringinaccuracies in Zlihe,m01lnting off the gun It; with respect to the .axialline vbif .the screen 13 .of the device. "Thetube J] is mollntedbehirid a transparent viewin plate .IA with Ith scr en 3 adja ent themtoand .the .nlate is Imounteicll in .an

:npeningof ,a panel 155 of .-.a casing enclosing the "tube and other .QOQnerating.,apparatus. Since the permanent magnet jocussingdeviee in is rigidly mounted .abo tithe. neck of thelt ewremoteirbm .theirent anelofthe casing. convenient dillst} .mentioif the elements of ,the .jiocussing fievlcejs accomplished by 'flex blecables ll 6 coupled to ,the

,Denmanentmagnetdevice andextendingthrough openings in-the lrontpanei l5. ":Ehese cab 1es.are

connected to knobs [1, of the control for varying the flux distribution of the focussing magnet structure and for adjusting the position of the beam projected to the screen of the tube and the viewing plate l4.

One embodiment of the focussing and centering magnetic structure of this invention is shown jinrEigs. ,2 e0 4, inclusive, and comprises a cylinildrical jcorezmember I8=of .magnetic 'material ha-ving high flux density and magnetic retentivity.

- gIihe magnetic properties are highly resistant to change by stray fields, vibration, or temperature and -such preperties are characteristic of mag- ;netic .allqys a: .-.-the .iron-nickel-alurninum type. 1A rnarti-cularly :desirable magnetic alloy of this :iiype is iAlriicoL-Noif2 having a composition of 17 per cent nickelhl'l per cent aluminum, 12 percent cobalt, "6 per cent copper and the balance iron. The ipfirmanent magnet core forms the main component of the device for focussing of the been set the .eathoderray tube rand tits imagnetic ,7

properties are; substantially -.constant. over a cansiderable Period of operation "due to .its stable characteristics and concentrated (flux field, ior ,instance, the magnetism '-.0f such ian-allo does not'chan-ge more than2 perrcentiover aztemper'ature range irom 1-40 .to ,1,00.0;F.-..and the fatigue loss due tostray fieldsand vibrations-is extremely .low. The core zmemloerz isclamped between a top ring pole-piece -19 and 'a .bottomiring mole-piece '26 which has aslightly larger diameter than the top pole-piece, the 'tWOiZDOlE-PlBCGS being secured together by screws :Z-IweXtending therebetween exterior to'thecore'member.

in order to "adapt the focussin'g :de ee ito. :the focussingof the ibeamtihgagcathode-ray'tube over 'a .ilarget'range :of (operating *voltages and to :vary the flux distribution *of the permanent magnet structure, aicylin'dri'cal sleeve orsring shunt member 2? of magnetic material, such as steel, is mounted exteriorly to thewore member I 8 and coaxial therewith and "is r reciprocally movable in an axial direction to :vary the gap 23' between the "lower end o'fthe-sleeve andthe bottom pole-piece '20. The cylindrical shunt -"is"slid-ably :actuated bya pluralityof equally spacedlead screwm'ernhers :or: spindles' l l .which extend "through the 'top pole piece I 9 and are seated in the bdttom' polepiece intermediate the -magntbore 18 and the "shunt 22. These lead scre'ws engage follower blocks 25-rigid1y secured to the: inner 'sur f'ace of the shunt ring '22 so that rotation of the lead screws in the blocks causes the shunt ring toibe reciprocally movable with respect 'to the bottom pole pi'ece 20. as shown' in Fig. 5ajto var'y the flux=dens'ity of the gap 23 and thereby-increase or decrease the strength of t-he magnetizing force influencing the beam o'f electrons in the "cathoderay tube over 'an-oper-ating range "of anode voltages from 3,000 to 7,000 volts. Each rotatable spindle'is provided witha'pinion gear 215 and'the three spindles are rotated simultaneously by a chain drive i 21. "-Ihe-drivingspindle 24, as shown -inFig. '2 ,fis'provided with a knurledknob 28 for manuarop'erationand the remaining spindles are driven simultaneously by the chain driv 2-;

slidably control the spacial relation o'f the-shunt sleeve with respec'tto the magnet structure. v.It is, of course, understood that the knob: :28 may be removed and .a flexible icablefli coupled to the drive spindle to accomplish the reciprocalmovemer t of the shunt with respect :to the magnet structure. The shuntsleeve is also provided'with a plurality .of rectan lar rut-out nertions .29 t

to facilitate manipulation by an auxiliary magnetic member or ring 30 mounted in axial and abutting relation on the rear of the bottom pole-piece 20. The centering ring 30 is held in position on the rear pole-piece by a plurality of retainer plates 3| which are mounted on spacer blocks 32 attached to the periphery of the. bottom pole-piece and permit the centering ring to slidably move across the bottom pole-piece so that the large diameter opening 33 of the centering ring is eccentrically controlled with respect to the larger diameter opening in the bottom pole-piece. The adjustment of the centering ring 30 is controlled by a pair of cam members 34 and 35, shown in Fig. 3, which are mounted on the bottom pole-piece in the same plane as the centering ring, the cam member 34 being mounted adjacent the periphery of the pole-piece in opposed relation to the retainer plate 3|, as shown in Fig. 3, and the cam member 35 being mounted in similar relation but displaced 90 degrees in a counter-clockwise direction.

The movement of cam 34 slidably shifts the centering rin 30 diametrically transverse to the axis of the magnet structure and the movement of cam 35 shifts the ring perpendicular to the movement of the ring by cam 34, the cam surfaces contacting the periphery of the ring 30 and regulating the movement thereof by the rotation bf the cams. The force exerted on the centering ring 30 by cam 34 is counteracted by a repulsion loop sprin 36' having its ends mounted in the block 32 and the loop portionengaging a groove in the periphery of the centering ring 30. 'A similar lop spring 31 counteracts the force of the cam 35 and is mounted in opposed relation thereto adjacent the periphery of the centering'ring. The cam members 34 and 35 are controlled by rotatable shafts 3B and 39 which project through the pole-pieces intermediate the coremember l8 and the shunt sleeve 22 and are prevented from longitudinal movement by a bifurcated leaf spring 45 engaging a circular slot on the shaft, as shown in Fig." 4, the leaf spring being held, in position by a retainer block 4| securedtothe top polepiece, the block also preventing turning of the shafts. The cam shafts are individually rotatableby a knurled knob 42, as shown in Figs. 2 and 4, although these controls may be remotely operated by flexible shafts l6, as shown in Fig. 1. The adjustable magnet structure of this invention may be mounted in rigid position around the neck of the cathode ray. tube by suitable radial .arms 43 extending from the bottom pole-piece, as shown in Fig; 2. a

Amodified form of the invention is shown in ..Figs.-6 to 8, inclusive, in which the shunt sleeve is adjusted with respect to the bottom pole-piece by manual rotation of the sleeve with respect to the magnet structure and the centerin member is movable in transverse directions perpendicular tothe axis of the magnet structure by reciprocal controls extending radially from the bottom polepiece; The construction of this embodiment of a the invention may be more clearly understood from Fig. 8 which shows the elemental parts of the" structure in exploded-fashion with the-related components in superimposed relation, the. composite assembly being shown in Figs. 6 and '7. In this form of the invention, the permanent magnetcore member I8 is clamped between a cupshaped ring pole-piece 45 and a larger diameter flat ring pole-piece 46 by bolts 41 which extend through the pole-pieces on the exterior-oi! the core member l8. The shunt rin 22 surrounds thepole-piece 45 and magnet core and is provided witha pair of oppositely disposed angularly directed slots 48, through which extend screws 49 secured in the cup-shaped pole-piece 45, the shunt sleeve being adjustably movable with re.- spect to the bottom pole-piece by the rotation of the sleeve coaxially with respect to the rigidly mounted pole-piece 45, the loosened'screws 49. in the slots and the pole-piece permitting rotation of the sleeve and tightening of'the screws 49 maintaining the properadjustment of the sleeve with respect to the magnet structure, as shown in Fig. 9a. v r 1 The auxiliary centering member magnet on the rear of the bottom pole-piece is formed of a rectangular plate 513 having an enlarged central opening 5| and two opposed smaller diameter openings 52 at diagonally related corners of the plate and the remaining corners being cut-off, as shown at 53. The centering plate is also provided with a pair of elongated slots 54 arranged in perpendicular relation to each other intermediate the central openin 5| and the edges of the plate on opposite sides of one of the corner openings 52. abutting relation to the bottom pole-piece 46 on the rear surface thereof and limit pins 55,.one of which is shown in Fig. 8, project from the rear surface of the bottom pole-piece. These pins are locatedin the apertures 52 of the centering plate when placed in position on the pole-piece. The limit pins and the centering plate are subsequently covered by retainer plates 56 of segmental form having a V-shapedrecess 51 on the under surface thereof to permit slidable movement of the centering plate 50 with respect to the bottom pole-piece 46 but preventing movement away fromthe pole-piece 46, the retainer platesjbeing rigidly secured to the bottom pole-piece. The bottom pole-piece 46 is provided with a pairof rectangular recesses 51 adjacent to the slots'54 in the magnet plate '50 and a rigidly mounted V guideblock 59 is secured in the recess adjacent the periphery of the pole-piece to receivea radially directed lead screw 60 having a knurled knob 5| on the outer end thereof. The lead screw is reciprocally movable in the rigidly mounted guide block 59 and engagesa follower block 62 slidably movable in the recess 58 of the pole-piece, the

lead screw being retained in the follower block byan angle plate 53 which engages a slot 64 on the lead screw. The follower block 62 is provided with an extension 65 with a'rais'ed bar projection 66, the bar 66 forming a slidable shoe extending into the slot 54 of the centering plate 50' so that reciprocal movement-of one of-the lead screws adjusts the position of the centering plate 50 with respect to the axis'of the magnet structure across one diameter' of the pole-piece while :theshoe in the other slot slideslongitudinally therein; When the centering plate 50 is adjusted transversel'yto the other extreme, as shown-in Fig. 9, the previously slidable shoe is now stationary in the slot while the other shoe is slidable in its respective slot. 1 I

The focussing and centering magnetic structure of. this-invention, together with the adjust- The centering plate 56 is mounted in shift said beam on said able contro s produce ac urate control of. he

fie d. ir n vth of; the i cus ine. flux oi the erof theib amfis effective toracourate y positi n a spot on thejscreen of the tubewithin a circle of one and once-h lf inches; diameter n a five inchscreen. Furthermore, se con tru i ns facilitate. themounting orqthe. focussing structurein acompact manner around the neckoi the tube and enable the controls of the adjustable components of the focussing device to be accomplished in a facile manner and with a minimum or lost motion. The centering controls also permi-t; fine adjustment or the centering plate. on the rear of the magnet structure to compensate for minute irregularities. in the mountingof the gun, whereby positioning .of' the beam with re: spect to the axis of" the tube isobtained.

While the specific structures of this invention, as herein disclosed, relate to particular manipulative controls with respect to the adjustments of theshunt sleeve and centering plate of the devices,'it is, of course, understood that various modifications maybe made therein without departing-fr mthe-scope of-the invention as defined in the appended claims.

What is claimed is:

1. In combination, a cathode-ray device including means to develop a beam of electrons and a, fluorescent screen on which said beam impinges and a magnetic structure surrounding said device for focussing and centering said beam on said .screen comp ising an annular core member of high magnetic flux density, a metallic shunt sleeve surrounding'said core member, means for adjusting said sleeve with respect to said core mernber to vary the flux distribution, an annular metallic plate supported on one end of said core I member, and means adjacent said core member engaging said plate for transversely moving .11; whereby the axis of said plate is laterally displaced with respect to the axis of said core memb1..

1 In combination a. c t od -ray tnbehaving asc en on one d. and an l tr u at the other end lior projecting a beam toward said screen, and a beam fvocussing and centering magne ic stru t re, surroun i g a d ube between said/screen and u mp is n an nn r o inan nt, magn t, a. metallic' hunt s ee c l with sa d ma net, means .f r recipro ly adjusting saidsleeve with respect to said magnet in an ax al directio an annula metallic p ate latent said magne and am p r m an fo transversely adjusting said plate perpendicular to. the, axis .of said magnet and tube to initially screen to a centering point thereo I v 3. I n cgmbination, a cathodeeray, device includin insane( f r d e ping ab am o el ct n and 7 target. screen imp nged. y aid beam an a magnet c s ruc r surroun ng said d ice o fpcussing and ocn ersa d b am. on said creen compris ng an annu r cor mem r of high m netic .flux density, neb le-pi c s o opp e end o said ore member a m tanicfshunt s eeve-coa ia w h. dcore me er, a s i a i tine s d leeve ith. r spect o sa d po e pieces to. va y he. flux; d s ribution annular insulin. Plate. supported Joy-o e o sa d po epieces, and means carried by said supporting polepiece en a in said plate .ior transverse mo mant. thereof. w ereby. the. axis of; said plate is I ateral y d placed wi h: respe t to; he at said core member;

In c m ina i n, a ca h ay tube inclu ing means to develop a beam of electrons and a lu r s en s r n on which said eami pinea nd a magnetic structur surroun ng said. ube or i c ssine and cent ing said b am on s id scre n omp is g: an annul r o meinlo roi high magnetic flux density, ring p pi es Qfi includi g me to deve p ab m o lectrons- .a d; a iuor ent scr en on which aidbc n nhh ng s, and a mag, ic tr otu esur oundi a sa d ube. tor ioci ssine and cent r ng said eam n s re mpri i a a nu magnetic mem e h g. o eec s a p ite ends a hunt ring ur o nd ng a d member, me ns for slidablii a just ng sa d n wi resp ct. o said po e p e o a s par e c nte ing ri gof math netic material on one end of said member and coaxial therewith, and adjustable means fo .IEEQY: ins said c ering ring i two ransverse dire on at ig an s o each oth r h resnect t th axi isa d magnet c me b r- In om ation a ca hod a ub i cl d= ins to de el p a eam o el ct ns. and luo cent. scre n t ward which s d boa. s. directed, an a ma n t st uc urroun sa d tu e for o uss n and d finit y s ion n the s t; of sa d beam on s d sc een com r sin annular cor member oi hi h ma ne ic d nsit ring ol pieces attach to opposite ends o sa d co e m mbe a cy ndrical meta lic shunt coaxia with sa d coremember a us able means. engaging said shunt forreciprocal move: .ment thereof with respect to said pole-pieces to vary the. flux distribution, an annular auxiliary disc member of magnetic material adjac nt one of said pole-pieces and slidably movable in die rections perpendicular to the axis of said poleieoe, .cams disposed in the plane of said disc ends: of said core member, a cylindrical metallic shunt coaxial with said core. member, adjustable means engaging said shunt for reciprocal move.-

.ment thereof with respect to said poleepieces to vary the flux distribution, an annular auxiliary disc member of magnetic material adjacent one .of said p01e=pieces and slidably movable in .d-irec.-

tion p rpendicular to theaxis of id 12. 1.. i a,

. Learns disposed in-the plane er said dis me bee.

a, cainner te m an on. ad

through said pole-pieces and secured to said cams, and cushion spring members disposed on said pole-piece to counteract displacement of said disc member.

'8. Incombination, a cathode-ray tube including means to develop a beam of electrons and a fluorescent screen on which the beam impinges, and a permanent magnet structure surrounding said tube for focussing and centering the beam spot on said screen comprising an annular core member of high magnetic flux density, ring polepieces attached to opposite ends of said core member, a cylindrical metallic shunt coaxial with said core member, adjustable means engaging saidshu-nt for reciprocal movement thereof with respect to said pole-pieces to vary the flux distribution, an annular auxiliary disc member of magnetic material adjacent one of said polepieces and 'slidably movable in directions perpendicular to the axis of said pole-piece, cams disposed in the plane of said disc member, said cams being adapted to alter the position of said disc member eccentrically with respect to said polepiece, cushion spring members disposed on said pole-piece to counteract displacement of said disc member, and retainer means supporting said circling said core, rotatable members extending through said pole-pieces and engaging said blocks for reciprocal movement of said shunt member with respect to' said core, pinions on one end of said rotatable members, and a chain drive engaging said pinions to actuate said rotatable members simultaneously.

10. An electron beam focussing and centering device for a cathode-ray tube having a target screen receptive to the beam comprising top and bottom ring pole-pieces, a cylindrical permanent magnet core coaxially clamped: between said polepieces, an auxiliary ring slidably. movable across the rear surface of the bottom pole-piece, a pair of retainer plates attached to said bottom polepiece extending over said ring, a pair of repulsion springs bearing against saidplatesand ring, and individually rotatable cam members in'contact with the periphery of said ring in oppositely disposed relation to said springs.

11. In combination, a cathode-ray tube .including means to develop a beam of electrons and a fluorescent screen impinged by said beam to forma spot'thereon and a permanent magnet structure surrounding said tube for focussing and positioning the spot on said screen comprising an annular core member of high magnetic flux density, ring pole-pieces on opposite ends and clamping said core member therebetween, an auxiliary plate having a central opening mounted adjacent the outer surface of one of said polepieces, guide blocks attached to said pole-piece adjacent two sides of said plate, and reciprocal screw members extending through said blocks engaging said plate for slidable movement thereof in two transverse directions perpendicular to the axis of said pole-pieces and core member.

12. In combination, a cathode-ray tube including means to develop a beam of electrons and two directions transverse 10 a fluorescent screen onwhich the beam impinges as a sharp spot, and a permanent magnet struc5 ture surrounding said tube for focussing and centering said beam on said screen comprising an annular core member of high magnetic fiux density, ring pole-pieces on opposite ends and clamping said core member therebetween, an auxiliary plate having a central opening and a pair of elongated slots therein in transverse directions along the sides thereof,-said plate being mounted adjacent the outer'surface of one of said pole-pieces, slidable means held' in'isaid slots, and radial feed means engagingsaid'slidable means to displace said plate reciprocally'in to the axis of said core member. f 13. In combination, a'cathode-ray tube includ ing an electron beam source and a fluorescent screen receptive to the beam to form a spot thereon, and a permanent magnet structure surround ing said tube for focussing and centering said beam on said screen comprising an annular core member of high magnetic flux density, ring pole-' pieces clamping said core member therebetween, an auxiliary plate having a central opening therein, elongated slots in transverse directions along the sides thereof, slidable means held in said slots, rotatable feed members extendin from said pole-piece in ,a radial direction andv on, and a permanent magnet structure surrounding said tube for focussing and centering said beam on said screen comprising an annular core member of high magnetic flux density, topand bottom pole-pieces clamping said core member therebetween, an auxiliary plate having a central opening therein and elongated slots in transverse directions along the'sides thereof, said plate ,being mounted adjacent the outer surface of said bottom pole-piece, slidable means held in' said slots, rotatable feed members supported on said bottom pole-piece and engaging said slidable means to displace saidplate reciprocally in two directions transverse to the axis of said core member, a metallic shunt sleeve surrounding said core member, said sleeve having angul arly directed slots therein, and locking means extending through said .slots fastened to'saidt'oppol'epiece.

4 15. In combination, a cathode-ray 'tubein eluding means to develop a beam'of electrons; and

a fluorescent screen on which the beam pinges, and a permanent magnet structure sure rounding said tube for focussing and centering the beam on said screen comprising an annular core member of high magnetic flux density, top and bottom ring pole-pieces clamping said core member therebetween, an auxiliary plate having a central opening and elongated slots on two sides of each opening perpendicular to each other, said plate being in contact'with said bottom polepiece, retainer blocks having recesses therein extending over portions of said plate and being secured to said bottom pole-piece, guide blocks adjacent opposite ends of one of said retainer blocks and afiixed to said bottom pole-piece in relation to said slots, threaded spindles extending through said guide blocks in transverse radial 

